I've a same problem, and when I play fallout shelter for several days and my battery is 100%, it magically drains to 41% of battery when I when I disconnect my charger, restart my tablet and connect the charger again.
Model: Vodafone Smart Tab 2
Regards: Modiken Silvia Mapeka
Screenshot below
HOW TO MAKE YOUR ANDROID SMARTPHONE'S BATTERY LAST LONGER
How Android batteries work
First, some background: most smartphones have either a lithium-ion battery or a lithium-polymer battery. Both are actually lithium-ion though, and as such, do not have a 'memory', which means you can charge them from any level – you don't have to fully discharge them before charging them up – and you don't have to charge them all the way to 100 percent.
So, try to adjust the settings of your device to save more battery.
1.) Black wallpaper can increase battery
2.) Doze Mode
3.) Turn off Google hotwords
4.) Don't let your apps fall behind the times
5.) Use Greenify
6.) Don't use adaptive/auto brightness
7.) Turn off vibrate and haptic feedback to save battery
8.) Set your 'Do Not Disturb' or 'sleep' schedule
9.) You don't need to be connected 24/7
10.) Don't get bogged down by widgets
11.) Explore the battery saving features on your phone
12.) Don't fall into the auto-sync trap
I hope, this method will help you !
